1. Field of the Invention
The present invention relates generally to a speech signal compression method. More particularly, it relates to a method for compressing a speech signal by using similarity of the F.sub.1 /F.sub.0 ratios in pitch intervals within a frame.
2. Description of the Prior Art
A main feature of speech coding methods for the transfer of a speech signal is to process the speech signal taking into consideration data transmission and compression rates for the transfer of the speech information, data transmission and compression rates for the transfer of the speech signal, the quality of synthetic speech, and the processing speed. In particular, speech compression methods based on linear predictive modeling occupies most studies.
In such methods, an input speech is passed through a low pass filter and analog/digital (A/D)-converted by an A/D converter. A linear predictive coding (LPC) analysis is performed with respect to the resultant digital signal to extract a pitch therefrom if it corresponds to voiced speech. FIG. 1 shows the construction of a speech coder (vocoder) based on the linear predictive model. Parameters such as the extracted LP coefficient, pitch, and energy are coded by a coder and transmitted through a communication channel or stored in memory for synthesis. Then, the transmitted or stored parameters are decoded by a decoder and synthesized by a synthesis filter.
The pitch is generally a derived signal based on a predictive error signal correlation, speech signal low-frequency analysis correlation, average magnitude difference function (AMDF), or cepstrum. However, the LPC analysis is inappropriate in such a case as a nasal speech where zeros, as well as poles, are needed in the transfer function, because it uses an all-pole model. Further, the LPC analysis cannot satisfy a variety of voice variations in that a speech source is dualized into a pulse train or a white random Gaussian sequence. Moreover, it is difficult to make a distinction between voiced and unvoiced speech and to accurately detect the pitch.